Researchers say they have developed a new and better method of freezing human sperm for later use in pregnancy attempts.

Researchers say they have developed a new and better method of freezing human sperm for later use in pregnancy attempts.

The new technique could potentially improve in vitro fertilisation treatment, and perhaps make it possible for HIV-positive men to donate sperm safely, the researchers say.

The freezing approach used in the study is already used for embryos and eggs, "and this is the next step, so it is logical," said Dr Ian Cooke, emeritus professor of obstetrics and gynaecology at the University of Sheffield in England, who is familiar with the study.

"Any improvements in sperm freezing would be welcome," said Mathew Tomlinson, a fertility specialist and scientist at Nottingham University Hospitals in England. "We can store sperm for many years, but only 25 to 30% of sperm survive from even the best samples."

Among cancer patients who want their sperm stored before they undergo chemotherapy, as little as 5% of sperm may survive, he added.

80% sperm survive freezing

When study lead author Raul Sanchez of La Frontera University in Chile and colleagues tested their alternative freezing approach, known as vitrification, they found that almost 80% of sperm remained viable after thawing.

In the traditional approach, sperm is frozen slowly and stored in liquid nitrogen. Vitrification involves removing the plasma in sperm, suspending the sperm in a sucrose solution and fast-freezing it in liquid nitrogen. It is then stored in liquid nitrogen or another kind of deep freeze.

This process results in greater sperm vitality and motility, and is less damaging to sperm, the researchers explained in a news release from the International Federation of Fertility Societies.

Because the sperm plasma - potentially home to the virus that causes Aids - is extracted, the researchers think their approach may also allow HIV-positive men to donate sperm without passing on the illness to a mother or baby.

But Dr Robert D. Oates, professor of urology at Boston University School of Medicine, said he's skeptical of the researchers' claims. The findings don't "translate directly into anything related to whether this allows better pregnancy rates," he said.

Moreover, the research doesn't prove that the freezing approach leaves the sperm free of disease, he added. "This is a technique that may have an application," Oates said, "but their claims are way overstated."

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